An inverter circuit is one kind of power converter. The inverter circuit is widely applied to an uninterruptible power supply system, a renewable power supply system or a backup power supply system. By alternately changing the on/off states of the switching elements of the inverter circuit, the inverter circuit can convert DC power into AC power.
FIG. 1 is a schematic circuit diagram illustrating a conventional full-bridge inverter circuit. As shown in FIG. 1, the conventional full-bridge inverter circuit 1 receives a DC input voltage Vin, and outputs an AC output voltage Vo and an output current Io. The full-bridge inverter circuit 1 comprises four switch elements S1˜S4, an inductor L, four body diodes D1˜D4 of the switch elements S1˜S4, and four antiparallel diodes D5˜D8. The upper switch element S1 and the lower switch element S2 are connected with each other in series to define a first bridge arm. The upper switch element S3 and the lower switch element S4 are connected with each other in series to define a second bridge arm. The node between the upper switch element S1 and the lower switch element S2 of the first bridge arm has a voltage VA. The node between the upper switch element S3 and the lower switch element S4 of the second bridge arm has a voltage VB. The antiparallel diodes D5˜D8 are electrically connected with an output terminal of the full-bridge inverter circuit 1 in order to reduce the influence of surge on the load.
FIG. 2 is a schematic waveform diagram illustrating associated signals processed by the conventional full-bridge inverter circuit of FIG. 1 by unipolar modulation. FIG. 3 is a schematic waveform diagram illustrating associated signals processed by the conventional full-bridge inverter circuit of FIG. 1 by bipolar modulation. The full-bridge inverter circuit 1 can be operated by unipolar modulation or bipolar modulation.
As shown in FIG. 2, the full-bridge inverter circuit 1 is operated by unipolar modulation. During the positive half cycle of the AC output voltage Vo, the switch element S1 is turned on, the switch element S2 is turned off, and the switch elements S3 and S4 are alternately turned on and turned off at a high frequency (e.g., higher than 1000 Hz). Meanwhile, the switch element S3 is used as a freewheeling switch element, and the switch element S4 is used as an energy-storage switch element. During the negative half cycle of the AC output voltage Vo, the switch element S1 is turned off, the switch element S2 is turned on, and the switch elements S3 and S4 are alternately turned on and turned off at a high frequency. Meanwhile, the switch element S4 is used as a freewheeling switch element, and the switch element S3 is used as an energy-storage switch element. That is, the magnitude of the AC output voltage Vo is changed between zero and a positive voltage value of the DC input voltage Vin during the positive half cycle, and the magnitude of the AC output voltage Vo is changed between zero and a negative voltage value of the DC input voltage Vin during the negative half cycle.
As shown in FIG. 3, the full-bridge inverter circuit 1 is operated by bipolar modulation. The on/off states of the switch elements S1 and S4 are synchronous, and the on/off states of the switch elements S2 and S3 are synchronous. The switch elements S1 and S2 of the first bridge arms are alternately turned on and turned off, and the switch elements S3 and S4 of the second bridge arms are alternately turned on and turned off. Consequently, the operations of the switch elements S1, S2, S3 and S4 during the positive half cycle of the AC output voltage Vo are respectively identical to the operations of the switch elements S1, S2, S3 and S4 during the negative half cycle of the AC output voltage Vo. In bipolar modulation, the switch elements S1, S2, S3 and S4 are alternately turned on and turned off at a high frequency (e.g., higher than 1000 Hz). That is, the magnitude of the AC output voltage Vo is changed between a positive voltage value and a negative voltage value of the DC input voltage Vin during the positive half cycle or the negative half cycle.
FIG. 4 is a schematic simulated waveform diagram illustrating the output voltage and the output current processed by the conventional full-bridge inverter circuit of FIG. 1 under a light load condition. As mentioned above, the conventional full-bridge inverter circuit 1 is operated at the fixed switching frequency. When the conventional full-bridge inverter circuit is operated under the light load condition, the peak value of the current Io flowing through the inductor L and the backward current corresponding to the peak value of the AC output voltage Vo are larger. Consequently, the turn-on loss and the turn-off loss of the switch elements are increased. Moreover, since the conventional full-bridge inverter circuit 1 is usually operated in a continuous current mode, the switch elements S1, S3 and the switch elements S2, S4 are operated in the hard switch state. Under this circumstance, the switching loss is increased, and the overall efficiency of the conventional full-bridge inverter circuit 1 is reduced. Moreover, some other problems (e.g., generation of switching noise, parasitic oscillation and gate driving interference) possibly occur.
For solving the drawbacks of the conventional full-bridge inverter circuit 1, some approaches are disclosed. An approach uses an additional circuit. Another approach controls the carrier wave or the modulated wave. However, the fabricating cost is increased, and the circuitry design and control mechanism are more complicated.
Therefore, there is a need of provides a control method of an inverter circuit in order to overcome the above drawbacks.